[swarming] Don't crash when the bot runs inside a GKE pod.
[swarming] Don't crash when the bot runs inside a GKE pod.

GKE exposes a limited imitation of GCE metadata server. Swarming bot
expects to see keys that are not present there, and crashes. Instead
handle it more gracefully.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org
BUG=1350632

Change-Id: I3dc2d902f71be43191dc9a698d38a55ca0025b0f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3815181
Auto-Submit: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py